1. Field of the Invention
This invention relates to a burglar alarm device, and more particularly to such a device in which electromagnetic radiation and ultrasonic radiation are simultaneously emitted and Doppler frequency shifted in response to reflection by moving objects, and in which the Doppler frequency shifted reflections are detected and evaluated by a circuit which supplies a common evaluation signal when a ratio of the Doppler frequencies is determined within a prescribed tolerance range relative to a frequency ratio prescribed for the device, the ratio being determined by the reciprocal value of the wave length ratio of the emitted radiations.
2. Description of the Prior Art
It is known in the art to apply the Doppler principle in burglar alarms. In devices of this type, radiation is emitted by a transmitter and is reflected by objects, including people. The reflected radiation is picked up by a receiver and evaluated. If the reflection results from an object at rest, the frequency of the received radiation coincides with the frequency of the transmitted radiation. If, however, the reflection results from an object which is moving with a velocity component in the direction of the transmitter and/or the receiver, or away from the transmitter or receiver, then a Doppler frequency shift occurs in the received radiation.
Known burglar alarms, for example some of which are commercially available, utilize electromagnetic radiation in the X-band. The frequency utilized is, for example, about 9.5 GHz. Electromagnetic radiation of this type permits relatively good manipulation. It may be generated, for example, with a semiconductor Gunn diode; and the receiver is equipped, for example, with a Schottky diode. In individual cases at least, however, a device of this type which operates in the X-band has a particularly troublesome disadvantage, which is based on the characteristics of the electromagnetic radiation. More specifically, the electromagnetic radiation readily passes through walls, and particularly through windows; and when there is a reflection from a moving object, for example a person, it does not matter whether the person is moving in the area which is to be monitored with the device or, possibly, in an adjacent corridor, or also possibly, outside on the street. In order to remedy this disadvantage, a device of this type has been made so insensitive that it then, unfortunately, is no longer fully reliable for monitoring the desired area.
There are also burglar alarms available on the market which operate with ultrasonic radiation, for example in a frequency range of about 40 kHz, instead of with radio waves. An advantage of devices of this type is that, in comparison to devices operating with radio waves, they are technically less expensive and correspondingly less expensive to construct. Ultrasonic devices also have special disadvantages, however. One particular disadvantage is that the transmitted ultrasonic radiation is also influenced, for example, by moving air, for instance by attenuation fluctuations. Particularly in heated areas, however, air turbulence cannot be precluded. In order to avoid false alarms, the method selected in this case as well was to make the device very insensitive. On the other hand, for these and other reasons, ultrasonic devices have practically only been used for monitoring smaller areas, such as vehicles and mobile homes.
U.S. Pat. No. 3,846,778 discloses a burglar alarm device in which not only electromagnetic radiation, but also ultrasonic radiation is used for the detection of a moving object, this device, however, is constructed in such a manner that the radio waves are used for one monitoring zone and the ultrasonic waves for another, different monitoring zone. An alarm is provided when an intrusion is detected in either of the zones. Coincidence for a reaction of the section operating with electromagnetic radiation is not provided and is not a particularly good concept in that case, it cannot be carried out. In order to achieve a simplification for both sections of such a device, namely to be able to use one and the same electronic evaluation circuit for the radio wave section and for the ultrasonic wave section, it is provided that the frequencies of the radio wave and the ultrasonic wave are tuned to each other in such a way that both types of radiation have equal wave lengths in air.
British Pat. No. 1,386,233 discloses a burglar alarm device in which one section operates with electromagnetic radiation and another section operates with ultrasonic waves. In this device it is provided that an alarm is given only when there is coincidence, namely when an occurrence which is to be reported is simultaneously detected in both device sections. In this known device, the two sections are individually operated and are connected together at their outputs for signal emission.
In the device disclosed in the British patent, it is not guaranteed that a reaction of both sections is due to reflection from the same identical object. This device would, in particular, trigger an alarm if, for example due to the healing or ventilating system there is a movement of curtains, or even only an air turbulence in the monitored area, and simultaneously, outside of that area, for example in an exterior corridor, a person walks by the monitored area. As set forth above, ultrasonic radiation reacts to moving curtains and the like. Electromagnetic radiation, for its part, is easily capable of penetrating even a masonry wall, so that the object detected with electromagnetic radiation need not have appeared in the monitored zone at all.
A device similar to the structure disclosed in the aforementioned British patent is described in U.S. Pat. No. 3,727,216. In this device, a section operates with electromagnetic radiation and another section operates with an ultrasonic radiation and the sections are connected together with respect to coincidence. The ultrasonic section involves a complete device which emits a signal as its output when an output voltage signal exceeds a predetermined threshold value. With this signal, which does not contain information concerning the velocity of the originally detected, moving objects, an AND gate is controlled in such a manner that only when this threshold value signal is present does the AND gate permit a Doppler frequency signal of the electromagnetic radiation section pass through to an evaluation circuit. Even in this burglar alarm device, it is not guaranteed that the coincidence is due to a reflection from the same identical object. Here as well, the reaction of the ultrasonic section may be due to a moving curtain and the reaction of the electromagnetic radiation section can be caused by a person moving outside of the monitored area. Such a coincidence of two different occurrences is in no way infrequent.